Certain devices such as wafer defect scanners, laser printers, document scanners, projectors and the like often employ a collimated laser beam that scans across a flat surface in a straight line path. These devices employ tilting mirrors to deflect the beam to perform the scanning. These tilting mirrors may be, or may include, Micro Electro Mechanical Systems (“MEMS”) devices.
Common mirrors used in MEMS devices include a stator and a rotor, with the rotor or structures carried by the rotor being reflective. The stator and/or rotor are driven with a drive signal which results in the rotor rotating with respect to the stator, thereby changing the angle of reflectance of an incident light beam on the rotor. By oscillating the rotor between two orientations, an opening angle of the mirror is defined, and scanning of the light beam across the flat surface is accomplished.
It is desirable to be able to precisely control movement of the rotor. To enable that precise control, collection of accurate data about the current position of the mirror is important. The collection of such accurate data, and consequently the precise control of the mirror, has proven troublesome due to changes in temperature, changes to the light beam itself, and other factors. This can result in commercially undesirable performance of the device. Thus, there is a commercial desire for the development of accurate ways to measure data about the position of the mirror, and precise ways to control the position of the mirror, are desirable.